Beverage containers, and the like are commonly made by blow molding a parison, or preform, that is made from polyethylene teraphthalate (PET) material. Such PET reheat and blow preforms are commonly manufactured in an injection molding machine. Injection molding machines are typically equipped with a hopper that contains particulate thermoplastic polymer resin, usually in pellet form. The resin particles are fed to an extruder where they melt under the application of thermal and shear energy. The resulting molten resin is then fed to an injection nozzle and injected into a mold. Once the molded resin has set, or frozen, sufficiently, the resulting plastic article is ejected from the mold, and the process repeats.
Proper cooling of molded articles represents a very critical aspect of the injection molding process because it affects the quality of the article and the overall injection cycle time. Cooling is particularly critical in applications where semicrystalline resins are used, such as the injection molding of PET preforms. Lack of sufficient cooling, or too slow cooling, can lead to the development of crystallized regions in a preform. Typically, to avoid crystallinity problems, the PET resin must remain in the mold cavity space for cooling for a sufficient period of time to prevent formation of crystalline portions and to allow the preform to solidify before being ejected.
Two things typically occur if a preform is too rapidly ejected from a mold in order to reduce the cycle time of the injection process. The first is that the preform is not uniformly cooled. For example, the mold gate area is often crystallized. The gate area is prone to crystallization because the resin in the mold cavity space is still in contact with the hot stem of the hot runner injection nozzle during the cooling phase in the mold. Crystallinity in the mold gate area can weaken the quality of an article subsequently blown from the preform. Another critical area of a preform is the neck finish portion which in many instances has a thicker wall and thus retains more heat than the other portions. This neck portion needs aggressive post-mold cooling to prevent it from becoming crystallized, and to sustain further manipulations. Secondly, a preform ejected too quickly from the mold may be too soft and can thus be deformed during subsequent handling.
Many attempts have been made in the past to provide post-mold cooling. However, the prior art systems have not resulted in a significant improvement in the quality of the molded preforms or a substantial reduction of the cycle time. For example, U.S. Pat. No. 4,382,905 to Valyi discloses an injection molding machine where the molded preform is transferred to a first tempering mold for a first cooling step and then to a second tempering mold for a final cooling step. The tempering molds are provided with internal means for cooling the walls that contact the preforms. Valyi does not provide for cooling of the preforms during the transfer steps. The device taught by Valyi adds significant complexity to an injection molding machine and process, and increases the size of the molding machines.
U.S. Pat. No. 4,592,719 to Bellehache discloses an injection molding apparatus for fabricating PET preforms where molded preforms are removed from the injection cores by a transfer device. The transfer device includes vacuum suction to hold the preforms, and air absorption (convection) cooling of the outer surface of the preform. Further cooling is provided by circulating ambient air inside the preform and withdrawing it through a suction rod inserted into the preforms. The apparatus taught by Bellehache does not provide directed cooling to problem areas, such as the mold gate area, and suffers many of the disadvantages of the prior art, such as poor cooling uniformity, high cooling times, and a high potential for preform deformation.
U.S. Pat. No. 5,176,871 to Fukai teaches a post-injection preform cooling method and apparatus. The preforms are transferred to cooling tubes and cooling air is blown around the outside of the preform. Cooling cores are inserted into the preforms to prevent shrinkage. The cooling rods taught by Fukai do not provide localized cooling to prevent crystallinity, and merely increase the overall cooling of the preforms in the cooling tubes while preventing shrinkage.
U.S. Pat. No. 5,338,172 to Williamson discloses a closed circuit cooling system for post-mold cooling of preforms. The preform is inserted into a cooling tube, and a cooling probe is inserted inside the preform. A cooling fluid flows around the outside of the preform, and then circulates into the preform and is withdrawn though channels in the probe. This system uses expensive liquid nitrogen or carbon dioxide coolants, and does not provide targeted cooling to problem areas, such as the mold gate area.
Further reference is made to U.S. Pat. No. 5,085,822 to Uehara. Uehara teaches a blow mold that includes a stretch rod provided with cooling means. Generally, the preform must be heated, stretched and blown, and cooled in rapid succession. The cooling means in the stretch rod simultaneously provide the air to blow the preform and to cool it, but does not provide localized cooling to prevent the development of crystallinity in the original preform.
It is, therefore, desirable to provide a method and apparatus that can provide localized or targeted cooling to the areas that are prone to crystallinity. It is further desirable that this cooling be provided outside the mold cavity to decrease overall cycle time.